Diversity techniques have been used for many years to address signal fading and other phenomena that take place in radio communications systems. In general, two or more signals from a source may be received, processed in two or more channels and combined in an appropriate manner in order to reduce effects of fading and other phenomena.
Diversity techniques include at least four types: space, frequency, time and angle diversity. A space diversity system includes two or more antennas spaced far enough apart to yield signals which have different fading characteristics. In the other systems, carrier frequencies are spaced apart, time delays are incorporated or separately polarized beams are used toward the same end. Although the present invention is discussed primarily with respect to space diversity systems, it may be used with any appropriate diversity techniques or systems.
Space diversity systems have long been used in over-the-horizon or forward scattering ultra-high frequency systems. Many of those systems combine two signals which share a common frequency by adjusting the phase of each signal to be combined with respect to the other. The aim is to ensure that the signals are substantially time coincident and in a predetermined phase relationship and to obtain maximum additive effect when the signals are combined. Such a system is disclosed in U.S. Pat. No. 2,975,275 issued Mar. 14, 1961 to Adams. That patent is incorporated by reference.
Such phase control "techniques" include adjusting the frequency of an oscillator in each signal channel with respect to a control signal corresponding to the phase. difference between the channels in order to cause frequency, phase and time coincidence of the intermediate frequency signals to be combined. One such technique involves the use of a phase detector to compare the phase of the IF signal in each channel. The phase detector is coupled to an oscillator which drives the two IF signals into desired phase relationship for additive combining. This technique is disclosed in U.S Pat. No. 2,955,199 issued Oct. 4, 1960 to Mindes, which is incorporated by reference.
Other diversity techniques involve adjusting not only the phase of the signal in each channel with respect to the other channel, but also adjusting both signals with respect to a reference frequency. The first type of control is referred to in this document as "differential mode" phase control while the second is referred to as "common mode" phase control.
Types of differential and common mode phase control are disclosed in U.S. Pat. No. 3,201,692 issued Aug. 17, 1965 to Sichak, et al., which is incorporated by reference. The Sichak patent discusses the use of such techniques in a single sideband communications systems. It discloses a system in which a phase detector monitors the combined signal and controls oscillators in one or more signal channels. Similar common mode phase control techniques are also disclosed in U.S. Pat. No. 3,348,152 issued Oct. 17, 1967 to Laughlin, Jr. et al., which is incorporated by reference.
Space diversity techniques have proven useful in satellite communications. Satellite spin creates periodic signal dropouts at essentially one dropout per revolution of the satellite about its axis. Satellite communications also involve doppler shifts which are not encountered in earth antenna communications systems. Doppler shifts result not only from the orbital motion of the satellite with respect to the earth's surface, but also from the satellite mounted antenna spinning about the axis of the satellite and thus moving with respect to the receiving antennas on the earth. Satellite spin dropouts and shifts occur quickly and dramatically, so a satellite communication diversity system must be able to handle fast fades and rapid doppler shifts.
Signals received from the satellite in a space diversity system have essentially identical modulation formats so that their spectral contents differ only in phase and frequency. The signals presented to a combiner in such a system may exhibit differences not only in strength and doppler, but also in down converter frequency offsets, delays due to differing cable lengths and other differences such as from oscillator characteristics. Diversity techniques in satellite communications must address not only these concerns, but they also should provide tracking for common mode doppler effects and centering the combined signal in the desired passbands of each receiver.
Equally important in diversity techniques is the need to weight the signals to be combined in an optimal fashion. The signals may be weighted according to the output gain of the combiner in order to compensate for fading and to normalize the output to required levels. This first type of weighting, referred to sometimes in this document as "common mode" weighting, includes a technique known as "equal gain" combining. Equal gain combining is described in U.S. Pat. No. 3,631,344 issued Dec. 28, 1971 to Greenwald, which is incorporated by reference. It discloses a system in which the output of the combiner, the common IF signal, is monitored to generate an automatic gain control signal. This AGC signal is applied to the IF amplifier in each channel in an effort to assure a constant amplitude, common IF signal at the output of the combiner.
Because equal gain combining causes the weaker signal to contribute undesired noise to the common signal, however, techniques have been developed to weight each signal to be combined with respect to the gain of the other signal. Such techniques are referred to in this document as "differential mode" weighting techniques. One differential mode technique has been referred to as "maximal ratio" combining. There, the gain for each IF signal prior to combining is adjusted according to the ratio of the signal levels of the channels. Maximal ratio combining is disclosed in U.S. Pat. No. 3,195,049 issued July 13, 1965 to Altman et al., which is incorporated by reference.